In conventional lithographic printing, ink receptive regions, known as image areas, are present on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise a layer of an imageable composition applied over the hydrophilic surface of a substrate. The layer of imageable composition typically comprises one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. If, after imaging, the imaged regions of the layer of imageable composition are removed to reveal the underlying hydrophilic surface of the substrate, the precursor is positive working. Conversely, if the unimaged regions are removed, the precursor is negative-working. In each instance, the regions that remain (i.e., the image areas) are ink-receptive, and the revealed regions of the hydrophilic surface accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imageable elements useful as on-press developable lithographic printing plate precursors have been disclosed in the literature. Such elements can be directly mounted on a press after imaging and developed with ink and/or fountain solution during the initial press operation. A separate development step before mounting on press is not required. On-press developable lithographic printing plate precursors are discussed, for example, in Teng, U.S. Pat. No. 6,071,675, column 2, line 47, to column 3, line 17.
Imaging of the imageable element with ultraviolet and/or visible radiation is, typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque regions. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, dimensions of the mask may change slightly due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Despite the progress in conventional on-press developable plates and digital laser imaginable plates, there is a desire for a lithographic plate precursor that can be imaged by infrared laser, does not produce ablation debris, and does not require a separate liquid development process. More specifically, there is a desire for thermally imageable elements that are on-press developable with ink and/or fountain solution.